Novel superconducting material

ABSTRACT

A superconducting material consisting essentially of the intermetallic compound represented by a general formula: (Nb1xTax)k Al1-yMy, wherein M is selected from the group consisting of Be, B, Si, and C, and the values of x, y, and k are 0 &lt; OR = x &lt; OR = 0.1, 0.01 &lt; OR = y &lt; OR = 0.2, and 2.3 &lt; OR = k &lt; OR = 4.0, respectively. This material has a higher critical temperature than that of the previously known Nv3Al.

United States Patent 11 1 Kawabe et al.

[111 3,884,683 1 1 May 20, 1975 1 1 NOVEL SUPERCONDUCTING MATERIAL [75]Inventors: Ushio Kawabe, Hamura; Shigeo Fukase; Masato lshibashi, bothof Hachioji; Mitsuhiro Kudo, Hamura; Kazue Takatoku, Kokubunji, all ofJapan [73] Assignee: Hitachi, Ltd., Japan [22] Filed: Jan. 24, 1972 21Appl. No; 220,211

[30] Foreign Application Priority Data Jan. 22, 1971 Japan 46-1584 [52}US. Cl. 75/174; 148/32; 148/325; 148/133; 335/216 [51] Int. Cl........C22c 27/00; C22f l/18; H0lf H04 [58] Field of Search 148/32, 133, 32.5;75/134, 75/174; 335/216 [56] References Cited UNITED STATES PATENTS3,275,480 9/1966 Betterton et a1 148/2 OTHER PUBLICATIONS Journal ofApplied Physics, Vol 37, No. 6, May 1966, pgs. 2218-2223.

Z. Naturforsch. 1035-1039.

Swartz et al. 148/133 X 26a, March 26, 1971, pgs.

Primary ExaminerC. Lovell Attorney, Agent, or Firm-Craig & Antonelli[57] ABSTRACT l4 Claims, 10 Drawing Figures HIMEB Mix? 2 0 i935 SHEET10F 6 FIG.

UNIT CELL OF B-W TYPE CRYSTAL STRUCTURE l NOVEL SUPERCONDUCTING MATERIAL2 where w,, is the average frequency of the phonons that scatterelectrons at the Fermi surface and e is the products of the netattractive interaction between the electrons energy (V) by the densityof states of d- The present invention relates to a superconducting 5electrons [N at the Fermi surface. material having a novel chemicalcomposition. and Then. in the B.C.S. theory the value of said w is moreparticularly to a superconducting mter represented as an approximationwith a value which is compound having a B- type cry tal SIIUCIUre andthe proportional to an order of the Debye temperature of high Crlll atemperat rethe superconducting material, the value of said V is a 2.Descript on O the or t constant, and said [N represented as anapproxima- Superconductmg materials have such interesting tion with avalue which is proportional to the electronic PPp as Peffectconductlvltyi dlamagnetlsmv transpecific heat coefficient (7/) of thesuperconducting sition phenomenon, etc. aterial.

Various appliances utilizing these properties, s 85 Therefore, in orderto obtain a superconducting masuperconducting magnets. magneti Shields,Y l5 terial having a high critical temperature, it is necessary and h kha hitherto been developed to find a material with high Debyetemperature and Generally, in order to reveal the superconductivity, hih electronic ifi heat coeff cient.

It necessary to cool the P{P matenal at Table 1 shows the criticaltemperature and crystal 3 lower temperature than 115 cl'mcaltemperamfestructure of some well known superconducting materi- TheCritical temperature 0f a i Whlch h als that have the considerably highcritical temperabeen considered to be the superconducting materialtux-e,

TABLE 1 BINARY COMPOSITION COMPOSITION CRITICAL CRYSTAL TEMPERATURESTRUCTURE Nb sn 17.8 18.3 [3 -W Nb Al 17.5 18.4 do. V -,Si 16.4 17.1 do.V;,Cre 17.0 do. V Ga 14.2 16.8 do.

NbN 11.0 16.0 NaCl TcMo 11.0 15.8 do. Nb Ga 14.2 p w MOC 9.0 13.5 NZICITaC 9.0 l 1.4 do.

TERNARY COMPOSITION COMPOSITION CRITICAL CRYSTAL TEMPERATURE STRUCTUREl7.8 NaCl having the highest critical temperature, is 18K.

Consequently, a cryogenic technique which utilizes expensive liquidhelium as a coolant is necessary in order to cool these superconductingmaterial below its critical temperature.

Under these circumstances the discovery of a superconducting materialhaving even a little higher critical temperature than that of the knownsuperconducting material has been greatly desired.

According to B.C.S. theory [J. Bardeen, et al: Phys. Rev. 108, l 175(1957)] relating to superconductivity, the critical temperature Tc ofasuperconducting material is represented by the following formula;

Some of the superconducting materials having the relatively highcritical temperature are found in a group of interrnetallie compounds,which have the B-W type crystal structure, the relatively highelectronic specific heat coefficient ('y), and the high Debyetemperature (61)), i.e., Nb Sn, Nb Al, Nb Al Ge and the like.

However, such superconducting materials having the B-W type crystalstructure as mentioned above cannot be obtained until they are subjectedto difficult ageing such as 700C X 1,001) hours. Thus the manufacture ofthese materials is not easy.

0n the basis of the McMillans theory presented recently in Phys. Rev.167 (68) 331, by W. L. McMillan,

the critical temperature of a superconducting material is given by thefollowing formula;

where w is the average frequency of the phonons, p. and A are theelectron-electron and electronphonon coupling constants, respectively Inthis Mc- Millan's theory, it is concluded that the maximum value ofcritical temperature of superconducting materials is within a range of25 to 40K.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a superconducting material having a novel composition which canbe made into a stable B-W type crystal structure having a high criticaltemperature even by a relatively short period of treatment in contrastto the previous superconducting materials having relatively highcritical temperatures.

Another object of the present invention is to provide a superconductingmaterials having a high critical temperatures and relatively highcritical current densities (.lc).

Another object of the present invention is to provide a superconductingmaterial to be used under simpler cryogenic conditions than previously.

The superconducting material due to the present invention is obtainedfrom an improvement of Nb Al which is previously well known for one ofthe superconducting materials having a high critical temperature in agroup of the superconducting binary intermetallic compound with the B-Wtype crystal structure.

The superconducting materials of the present invention are characterizedin that a suitable amount of Al of the Nb Al is substituted by atertiary element M, which has a smaller atomic radius of thecoordination number 12 than that of Al, and which simultaneously has ahigher Debye temperature than that of Al, respectively.

According to the present invention, the abovementioned tertiary elementis an element selected from the group consisting of berillium (Be),boron (B), silicon(Si), and carbon (C).

Occasionally, according to the present invention a suitable amount of Nbof the material NbgAl is further substituted by tantalum (TA).

Therefore, the chemical composition of the superconducting material ofthe present invention is represented by the following general formula;

( 1-J J)k l-.u w

where the values of x, y, and k are selected within such ranges that itis easy to crystallize the composition in a desired B-W type crystalstructure and hence heat treatment is also easy.

Accordingly. the suitable ranges of said values of x,

y, and k are given as follows:

0 5 x 0.05, 0.0l y 5 0.2, and 2.3 5 k S 4.0

BRIEF DESCRIPTION OF THE DRAWINGS The superconducting material of thisinvention will be further understood by reference to the followingdetailed description and the accompanying drawings wherein:

FIG. I is a schematic model of the B-W type crystal structure;

FIG. 2 is a graph showing the relation between the concentration of Beand the critical temperature for the material as cast" and as aged" ofthe superconducting Nb Al Be series;

FIGS. 3, 4 and 5 are graphs showing the relation between theconcentration of B, Si, and C and critical temperatures for as cast" andas aged" of superconducting Nb Al ,B,,, Nb Al, ,,Si,,, and Nb Al,.,,C,,,respectively;

FIG. 6 is a graph showing the relation between the critical temperatureand the ageing temperature of 3 o.9s 0.0s and a dss ncs' FIG. 7 is agraphshowing the relation between the critical temperature and theageing time of Nb Al 5 0.05 and a o.95 o.s;

FIGS. 8 and 9 are graphs showing the relation between the concentrationof Be and B and the critical temperatures for as cast and as aged"superconducg i oss aosla w m and t).95 0.05)3 l-u y series,respectively;

FIG. 10 is a graph showing the I-I-Jc curve of Nb Al Be Nb Al B andpreviously known Nb Al Ge DESCRIPTION OF THE PREFERRED EMBODIMENTS Amodel (unit cell) of the B-W type crystal structure of the intermetalliccompound represented by a chemical formula Nb Al shown in FIG. 1. It ischaracteristic of this crystal structure that it is constructed by thecomplex lattice structure comprising a body centered cubic crystallattice consisting of Al and the chain like lattice consisting of Nbatoms situated on the three faces of said body centered cubic latticecrossing rectangular each other.

In this superconducting material having the B-W type crystal structure,it is generally known that the critical temperature increases when thecomposition of the material approaches the stoichiometric compositionand that the chains of Nb atoms form extremely narrow dband structure atthe Fermi surface in the B-W type crystal structure.

Since the d-band structure as aforesaid and the density of state ofd-electron at the Fermi surface are increased, the critical temperatureis thereby increased.

The present invention is based on the discovery by the present inventorsthat when the Al atoms of the NbgAIill'C substituted by the atoms ofaforementioned tertiary element M, the Debye temperature is increasedand the lattice constant (a as shown in FIG. 1) is decreased, and thatthe material can be thereby obtained with a higher critical temperaturethan the base compound Nb Al.

Furthermore. the present invention is based on the discovery by thepresent inventors that when a suitable amount of Nb of theabove-mentioned superconducting material is substituted by Ta inaccordance with the present additional invention, the B-W type crystalPreparation of Specimens As raw materials, all samples of elements Nb,Ta, Al, and tertiary element M each having a purity of 99% up wereprepared, and weighed quantities of each for various desired values ofx, y, and k of the chemical formula (Nb .,Ta,) (Al, ,,M,,) were meltedunder an argon atmosphere in a plasma arc furnace, the melts wereinverted for several times to mix them uniformly, and the melts weresolidified into button like samples. The materials thus prepared wereagain melted using a leviation melting furnace, and then cast intowatercooled copper mold to form a rod shape ingot (this is an as castspecimen) of about 3 mm in diameter and about 30 mm long. Casting isconducted in argon atmosphere.

The "as cast" specimen was placed in a quartz tube, sealed in a highvacuum and then aged at a temperature of 650 to 1100C. for 24 to 360hours. Generally the high vacuum is equal to about l mm Hg andpreferably from about mm Hg.

The values of x, y, and k of the resulting intermetallic compounds werequantitatively determined by a chemical analysis, and the crystalstructures and lattice constants thereof were examined by an X-raypowder diffraction method.

According to the result of the X-ray analysis, some of the as cast"specimens showed X-ray diffraction patterns of the B-W type crystalstructure, which patterns were observed to be broad.

As the heat treatment proceeded, the patterns become sharp. Particularlyeffective heat treatment is done in a temperature range of from about600 to about 700C. for a period of time equal to at least about 50 hoursand preferably equal to 300 hours and in some cases longer, e.g. 500hours.

From the result of the above-mentioned investigations, it has beenconfirmed that (Nb Tafl Al M in which the values of x, y, and k areselected within ranges of 0 5 x 3 0.l, 0 S y S 0.2, and 2.3 S k 5 4.0,respectively, result in a favorable B-W type crystal structure by virtueof the above-mentioned heat treatment.

According to the present invention, more particularly, when the valuesofx and y are within a range of O 5 x :1 0.05 and 0.01 i y 5 0.2 andwhen the value of k is about 3, that is, the chemical compositioncomprises about 70 75 atomic of Nb, about 0 4 atomic of Ta, about 24.75atomic of Al, and about 0.25 5 atomic of the tertiary element M, thematerials have relatively high critical temperatures as compared withthe previously known compound Nb Al.

Superconducting Properties of the Specimens The critical temperature wasmeasured by a conventional four-probe resistivity technique when acurrent density of l A/cm passed through a specimen of 30 mm long. Thecritical temperature was determined to be a temperature at which theresistivity of the specimen became one-half the difference betweenresistivities of the superconducting and normal states during thetransition.

The thermometer used in that measurement of the critical temperature isa germanium thermometer by Honewell Co. in US. which calibrated threetemperatures of liquid helium, liquid hydrogen, and liquid nitrogenunder an atmosphere.

FIG. 2 shows the relation between the concentration of Be and thecritical temperature for as cast" and as aged" of the material Nb Al Bei.e., the value of x is set at 0 and the value ofk is set at 3 subjectedto ageing at 700C for 300 hours.

These relations for the as cast" and as aged are substantially similarto each other, the critical temperature of the as aged" being higherthan that of the as cast" specimens by about 2C.

As seen from FIG. 2, when the concentration of Be is within a range ofless than about 5 atomic i.e., the value of y is within a range lessthan 0.2, the critical temperature of the present specimens is higherthan that of the base compound Nb Al, i.e., when y 0.

A peak of the critical temperatures occurs between the range of about1.2 to 3 atomic of the Be concentration.

FIGS. 3 and 4 show the relationship between the concentration of B andSi of Nb Al B and Nb Al, ,,Si,, and the critical temperatures of ascast" and as aged specimens subjected to ageing at 700C for 300 hours,respectively.

An FIG. 5 shows the relationship between the concentration of carbon ofNb Al C and the critical temperatures of as aged" specimen providedunder the same condition noted above.

As seen from these Figures, the relation between the criticaltemperature and composition of these specimens is similar to the resultof the previously mentioned Nb Al,.,,Be,,.

And then, the peak values of critical temperature of the presentexamples were 19.5K (M Boron; y 0.1), 18.6K (M Silicon; y =0.l5) and19.0]( (M Carbon; y 0.15).

FIG. 6 shows the relationship between the critical temperatures and theageing temperatures for 1 hour provided for Nb Al Be and Nb Al ,,B of asaged specimens, and for a reference this figure also shows the ageingrelation for 50 and 500 hours at a temperature range of only 600 to800C.

In the present investigations, a peak and a valley of the criticaltemperatures were observed at an ageing temperature range of 600 to 800Cand at a range of 1,000 to 1,500C, respectively.

FIG. 7 shows the relation between the critical temperature of as aged"specimens and the ageing time of Nb Al Be and NbsAl ogsBq os at thetemperature of 700 and 800C.

As seen in FIG. 7, the critical temperature increases with increasing ofthe ageing time and a leveling off after more than about hours areobserved.

It is therefore clear according to the present superconducting materialthat the ageing time necessary for obtaining high critical temperatureis very short as compared with that of the previous materials such asaforesaid NbaAlugGeo g and the like.

According to the present invention, the lattice constant of the B-W typecrystal structure of Nb Al ,,M,, is decreased upon an increase in thetertiary element M and the fi-W type crystal structure is caused to bemore stable by substitution of Nb with Ta which has smaller atomicradius than that of Nb and the interatomic distance of the chain latticeconsisting of Nb atoms is thereby decreased matching to a decrease ofsaid lattice constant a o Generally, the amount of Ta substituted for Nbis not more than about 5 atomic FIGS. 8 and 9 show the relationshipbetween the values of y and the critical temperature of (Nb Ta,, Al Beand (Nb =,Ta Al B of as cast and as aged specimens subjected to theageing of same condition as aforementioned specimens, i.e., 700C for 300hours.

Moreover, some of the superconducting material of the present inventionhas considerably higher critical current density (.lc) as compared withthe previous superconducting materials.

FIG. shows the critical current density (Jc) versus transverse magneticfield (H) properties of the present Nb Al Be Nb Al B and the previous NbAl Ge which have a high critical temperature.

The critical current density (Jc) is determined to be a current dividedby the cross-sectional area of the specimen when a voltage of I00 uV canbe detected across both ends of the specimen by an electric currentbeing passed at 4.2"K in the applied transverse magnetic field.

As seen in FIG. 10, the superconducting Nb Al Be and Nb Al B subjectedto the heat treatment have the current carrying capacity of 8 X l0 2 X10 A/cm at 4.2K even in a transverse magnetic field of 60 KOe. Thisvalues of the critical current density are fairly high as compared withconventional material, for example, Nb Al Ge which is known as thematerial having the highest critical temperature previously.

What is claimed is:

1. A superconducting material consisting essentially I (Nb TaQ Al M,

wherein M is an element Selected from the group consisting of Be, and Siand the values of x, y, and k are 0 S x 5 0.1, 0.01 S y S 0.2, and 2.3 Sk S 4.0, respectively.

2. The superconducting material of claim 1, wherein the composition ofthe material consists essentially of about 74 to about 84 atomic of Nb,about to about 29 atomic of Al, and about I to about l5 atomic of anelement selected from the group consisting of Be, and Si.

3. The superconducting material of claim 1, wherein the composition ofthe material'consists essentially of about to about atomic of Nb. about0 to about 4 atomic of Ta, about 20 to about 24.75 atomic of Al, andaout 0.25 to about 5 atomic of the element represented by M.

4. The superconducting material of claim 1, wherein k equals 3 and xequals 0.

5. A superconducting material consisting essentially of an intermetalliccompound having a B-W-type crystal structure represented by the formula:

where M is an element selected from the group consisting of Be, and Si.the values of x, y, and k are 0 5 x 5 01,001 5 y S 0.2, and 2.3 5 k S4.0, respectively, and said material has been heat treated for a periodof at least about 50 hours at a temperature of at least about 600C.

6. The superconducting material of claim 1, wherein said material hasbeen aged so as to have a high critical temperature.

7. The superconducting material of claim 1, wherein said material has acritical temperature greater than at least about l7.0K.

8. The superconducting material of claim 1, wherein said materialexhibits a critical current density of at least greater than 8 X 10 A/cmfor transverse magnetic fields of 30-80 KOe at 4.2l(.

9. The superconducting material of claim 1, wherein the value ofy is0.05 S y 5 0.2.

10. A superconducting material consisting essentially of anintermetallic compound having B-W type crystal structure represented bythe formula:

wherein M is Be in the amount of about 1.2 atomic and the values of x,y, and k are 0 i x S 0.1, 0.01

5 y 5 0.2, and 2.3 S k S 4.0, respectively.

11. The superconducting material of claim 1, wherein M is Si and y is0.15.

12. A superconducting material consisting essentially of anintermetallic compound having a B-W type crystal structure, wherein saidintermetallic compound is a n.ss o.os-

13. The superconducting material of claim 5, wherein the value ofy is0.05 S y S 0.2.

14. The superconducting material of claim 3, wherein said compositionincludes the element represented by M in an amount of about 1 to about 5atomic

1. A SUPERCONDUCTING MATERIAL CONSISTING ESSENTIALLY OF AN INTERMETALLICCOMPOUND HAVING A B-W TYPE CRYSTAL STRUCTURE REPRESENTED BY THE FORMULA2. The superconducting material of claim 1, wherein the composition ofthe material consists essentially of about 74 to about 84 atomic % ofNb, about 15 to about 29 atomic % of Al, and about 1 to about 15 atomic% of an element selected from the group consisting of Be, and Si.
 3. Thesuperconducting material of claim 1, wherein the composition of thematerial consists essentially of about 70 to about 75 atomic % of Nb,about 0 to about 4 atomic % of Ta, about 20 to about 24.75 atomic % ofAl, and aout 0.25 to about 5 atomic % of the element represented by M.4. The superconducting material of claim 1, wherein k equals 3 and xequals
 0. 5. A superconducting material consisting essentially of anintermetallic compound having a Beta -W-type crystal structurerepresented by the formula: (Nb1-xTax)k Al1-yMy where M is an elementselected From the group consisting of Be, and Si, the values of x, y,and k are O < or = x < or = 0.1, 0.01 < or = y < or = 0.2, and 2.3 < or= k < or = 4.0, respectively, and said material has been heat treatedfor a period of at least about 50 hours at a temperature of at leastabout 600*C.
 6. The superconducting material of claim 1, wherein saidmaterial has been aged so as to have a high critical temperature.
 7. Thesuperconducting material of claim 1, wherein said material has acritical temperature greater than at least about 17.0*K.
 8. Thesuperconducting material of claim 1, wherein said material exhibits acritical current density of at least greater than 8 X 103 A/cm2 fortransverse magnetic fields of 30-80 KOe at 4.2*K.
 9. The superconductingmaterial of claim 1, wherein the value of y is 0.05 < or = y < or = 0.2.10. A superconducting material consisting essentially of anintermetallic compound having Beta -W type crystal structure representedby the formula: (Nb1-xTax)kAl1-yMy, wherein M is Be in the amount ofabout 1.2 atomic %, and the values of x, y, and k are 0 < or = x < or =0.1, 0.01 < or = y < or = 0.2, and 2.3 < or = k < or = 4.0,respectively.
 11. The superconducting material of claim 1, wherein M isSi and y is 0.15.
 12. A superconducting material consisting essentiallyof an intermetallic compound having a Beta -W type crystal structure,wherein said intermetallic compound is Nb3Al0.95Be0.05.
 13. Thesuperconducting material of claim 5, wherein the value of y is 0.05 < or= y < or = 0.2.
 14. The superconducting material of claim 3, whereinsaid composition includes the element represented by M in an amount ofabout 1 to about 5 atomic %.